1. Technical Field
The present invention relates to restoring data. More particularly, the invention concerns restoring data and permitting access to the data while the data is being restored.
2. Description of Related Art
In modern computing systems, data is often backed up to provide protection from data loss if the original data becomes corrupted, and/or to archive the data. Magnetic or optical tape is often used for backing up data. Because the data is stored sequentially on the tape, restoring the data takes the amount of time required to read the tape from the beginning to the end of the data.
An application program may require access to data that has been backed up onto tape. However, before the application can use the data, the application must wait for the data to be retrieved from the backup. More specifically, when an application requests a file which must be restored from tape, the application must wait for the time required to restore the entire file. The delay is linearly proportional to the size of the file, and is exacerbated with large files. This delay is present when data is restored from a sequential data format, such as tape, to a direct access format such as a magnetic disk, and more generally is present whenever data is restored from one format to another, for example when compressed archived data is restored to an uncompressed format.
Although an application may require information from only a portion of a large file, with previously known techniques the application cannot be allowed access to the desired portion of the file until the entire file has been restored. This is the case because no facility exists to prevent the application from attempting to access the unrestored portions of the file, which would result in an error. Consequently, known techniques for restoring data from backup are inadequate for allowing speedy access to the data.